- Pollen grains land on a plant stigma.
- If the pollen is of the same species, a pollen tube begins to grow.
- The pollen moves down the pollen tube to the ovule.
- Fertilisation occurs when the nucleus from the pollen cell fuses with a nucleus in an
ovule. This produces a zygote.
- The zygote develops into an embryo plant.
Factors affecting seed germination:
● Water – water moves into the seed, causing it to swell. This allows the embryo to begin
● Oxygen – used in respiration to produce energy for growth.
● Temperature – the seed contains enzymes, e.g. for respiration, which will work faster at
the plant’s optimum temperature. This is why seeds are dormant in the winter and grow
again in the spring.
Sexual reproduction in humans
Male reproductive system:
● Testes – there are two testicles. This is where sperm cells are produced, as well as
● Scrotum – Contains the testicles.
● Sperm ducts – Tubes that carry sperm from the testes to the urethra.
● Prostate gland – Secretes nutritive fluid which combines with sperm to form semen.
● Urethra – Tube which allows excretion of urine and semen from body.
Female reproductive system:
● Ovaries – there are two ovaries. Their function is to develop egg cells. Woman have
undeveloped egg cells from birth, whereas men produce new sperm throughout their
● Oviducts – connect to each ovary and contain cilia to transport the egg cells through the
tube. This is where fertilisation occurs.
● Uterus – this is where the foetus develops.
● Cervix – separates the vagina from the uterus, and also holds the baby in place during
pregnancy. The cervix is made of muscular tissue.
● Vagina – Tube that leads from the cervix to outside of the body. Receives the penis
Fertilisation occurs when a sperm cell and an egg cell fuse their nuclei together. Sperm cells are
male gametes produced in large numbers in the testes. They are adapted by having a tail-like
flagellum which allows movement to the egg cell. Sperm cells also contain many mitochondria
to produce energy for this movement. Eggs, in contrast, are much larger than sperm and are
unable to move themselves. They are instead transported by cilia on the walls of the oviducts.
When the sperm cell reaches the egg cell, it must digest the wall of the cell so that it can fuse
their nuclei. This is done using enzymes located in the acrosome. The egg contains a jelly coat,
which ensures that only one sperm cell can enter.
Once fertilisation has occurred, the zygote undergoes mitosis (cell-division) to produce many
cells which make up an embryo. The egg cell contains energy stores to allow this to happen.
The embryo is implanted into the wall of the uterus, where it grows.
Development of the foetus:
● Umbilical cord – allows the exchange of substances between the foetus and the mother
through the cord.
● Placenta – Connects to the foetus end of the umbilical cord and allows exchange of
substances. It also produces hormones such as oestrogen and progesterone.
● Amniotic sac – Surrounds the foetus and produces amniotic fluid.
● Amniotic fluid – protects the foetus.
The mother passes essential nutrients to the foetus through the umbilical cord, such as amino
acids, oxygen and glucose. These help build cells, and hence structures, in the foetus. In
addition, waste products diffuse out of the foetus to be excreted from the mother’s body. This
prevents a build-up of toxins which could harm the foetus. In addition, antibodies are passed to
the foetus, allowing it to develop a resistance to pathogens.
Throughout this exchange of substances, the blood of the foetus and mother do not mix;
instead, substances diffuse between them. This is to prevent diseases passing to the foetus
through the blood, although some toxins, such as nicotine from cigarettes, and pathogens such
as the rubella virus, can still pass from the mother to the foetus.
Diet is also important during pregnancy. A greater amount of protein and carbohydrates
should be consumed to provide nutrients for the foetus to grow. Nutrients such as iron, which
is needed to make haemoglobin in blood, and calcium, which is used in bone growth, are also
essential to the foetus. Drugs, alcohol and cigarettes should be avoided as they can harm the
foetus. Babies whose mothers smoked or consumed alcohol during pregnancy are more likely
to have issues in cognitive development, behaviour and growth. There is also a larger risk of
Birth of the child:
- The amniotic sac breaks, releasing the amniotic fluid.
- Muscles in the uterus wall contract to push the baby out while the cervix dilates.
- The baby exits the mother through the vagina.
- The umbilical cord, which is still attached to the baby, is cut and tied.
Breastfeeding vs bottle-feeding:
● Breastfeeding allows antibodies to be passed from the mother to the child, providing
● Breastfeeding contains all the essential nutrients for the baby. Some nutrients in
bottled milk may be harder for the baby to digest. There is also no risk of an allergic
reaction to breastmilk.
● Breastfeeding may be painful for the mother.
● Breastfeeding can only occur when the mother is present.
Sex hormones in humans
Two hormones are key to the development of secondary sexual characteristics during puberty
and the subsequent regulation of these characteristics:
● Testosterone – Testosterone is produced in the testes of males, and in small amounts in
the ovaries of females. It is responsible for muscle development and the deepening of
the voice, amongst other things.
● Oestrogen – made in the ovaries of females. It leads to the development of widened
hips, breasts, and plays a part in the menstrual cycle.
The menstrual cycle happens approximately every 28 days. During each cycle, an egg cell is
released from the ovaries. The uterus wall thickens by filling with blood capillaries in
preparation for a pregnancy, which would occur if the egg is fertilised. If this egg is not
fertilised, the egg dies and menstruation occurs, where the dead egg cell and old uterus lining
is expelled from the body in a period.
The menstrual cycle is regulated by four hormones:
● FSH – Follicle stimulating hormone triggers the development of an egg cell in the ovary,
and also stimulates oestrogen production in the ovaries. This is produced in the pituitary
● LH – Luteinising hormone triggers an egg to be released, as well as stimulating
progesterone production in the ovaries.
● Progesterone – Progesterone is responsible for maintaining the thick uterus lining in the
cycle and during pregnancy. It also decreases FSH production.
● Oestrogen – Stimulates LH production, whilst decreasing FSH production.
At the beginning of the cycle, levels of FSH and LH are high to stimulate egg production and
cause the production of oestrogen which thickens the uterus lining. When the egg is released,
the levels of LH, FSH and oestrogen decrease, whilst progesterone is released to maintain the
uterus lining. If the egg is not fertilised, progesterone levels decrease and the uterus lining
breaks down, causing menstruation.
Birth control in humans
Birth control is used in family planning to control when, and how many, children are produced.
There are many methods to prevent pregnancy if children are not wanted, including:
● Natural birth control methods – these methods use the knowledge of the menstrual
cycle to avoid pregnancy. This is done by monitoring body temperature and cervical
mucus to predict when ovulation is occurring and avoiding sexual intercourse in this
period. As cycles can be irregular and difficult to accurately predict, this method is not
always reliable. Another natural method is to abstain from sexual intercourse.
● Chemical methods – chemical methods include the contraceptive pill, implant and
injection, as well as IUD and IUS. These contain hormones which stop pregnancy. The
contraceptive pill contains the hormones progesterone and oestrogen, which prevents
ovulation. The contraceptive implant, injection and IUS both release progesterone and
cause the uterus lining to thin, preventing the fertilised egg from implanting in the
uterus wall. IUD also prevents implantation of the egg by thinning the uterus lining, and
also acts as a physical barrier to stop sperm reaching the egg.
● Barrier methods – this includes the condom, femidom and diaphragm. These are used to
act as a physical barrier and prevent the sperm from reaching the egg. Condoms also
have the advantage of preventing the spread of STIs.
● Surgical methods – these procedures prevent the release of sperm and egg cells. A
vasectomy involves the sperm ducts being tied or cut, preventing sperm from exiting
the testes. In women, the oviducts can be tied or cut to prevent the release of egg cells
from the ovaries.
On the other hand, there are also people who would like to have children who have difficulty
becoming pregnant. In vitro fertilisation (IVF) and artificial insemination (AI) are two fertility
treatments which can solve this:
● IVF – If fertilisation cannot occur due to the sperm count or quality being too low, the
egg cell can be fertilised outside of the body and then be implanted back into the
● AI – sperm is directly inserted into the uterus. This sperm can be from the partner, or
from a sperm bank if their sperm is not of a high enough quality.
Social implications of contraception and fertility treatments:
Some people believe that fertility treatments, such as IVF, and genetic screening of embryos
may lead to ‘designer babies’, where parents will discard eggs which do not have desired
characteristics, for example a certain hair colour or gender. In addition, they may avoid having
a child with an inherited disease.
Some religious groups do not agree with artificial methods of contraception. This is because
they believe that they are preventing a life that otherwise would have been created, which
goes against their beliefs. Fertility treatments are also contentious as during IVF, multiple eggs
are taken and fertilised at once, leading to spare embryos which are afterwards frozen and
stored or destroyed.
Sexually transmitted infections
Sexually transmitted infections, known as STIs, are infections that are transmitted via bodily
fluids during sexual intercourse or contact. The spread of STIs can be controlled by abstaining
from sexual contact, avoiding risky sexual practices and always using protection such as
Human immunodeficiency virus (HIV) is an example of an STI. HIV is present in the bodily fluids
of infected people, such as blood and semen, and can be transmitted during sexual intercourse.
In the blood, HIV attaches to lymphocytes (white blood cells) and enters the cell. Here, it uses
the cell to replicate itself and thus the cell cannot carry out its normal functions. Consequently,
HIV reduces the number of functioning lymphocytes, as well as reducing the body’s ability to
produce antibodies to fight off infection. HIV leads to AIDS, which makes the person extremely
susceptible to other pathogens as the white blood cells can no longer fight off disease, which
can be fatal.
There are two types of reproduction.
Asexual reproduction and sexual reproduction.
Asexual reproduction: is the process resulting in the production of genetically identical offspring from one parent.
It occurs in bacteria fungi and tubers.
- Bacteria are tiny single-celled organisms. They reproduce by a process called binary fission.
- In binary fission, one bacterium grows and exact copy of its DNA coil which carries its genetic information. Then the bacterium completely divides which one DNA coil in the parent and one in the daughter bacterium.
- Each bacterium can undergo binary fission once every 20 minutes making them able to reproduce massive numbers from one parent in very little time.
- Fungi are multicellular organisms that grow long threads called hyphae on pieces of food.
- There are two types of hyphae, reproductive and feeding hyphae.
- Reproductive hyphae grow vertically above the food material. At the top of the hyphae, there is a spherical bag in which many spores are grown.
- This bag is called sporangium. Spores being produced in the sporangia are reproductive structures that can grow into another fungus. At some point, the sporangium will burst open dispersing the spores into the air. If a spore falls on an area of favorable conditions (food – water – air) it will germinate and grow into a new identical fungus.
Runners (stolons) and rhizomes:
- Some plants naturally produce side branches with plantlets on them.
- The spider plant does this. Other plants, such as strawberries, produce runners with plantlets on them.
- Rhizomes grows under the ground
- Many plants naturally develop underground food storage organs that later develop into the following year’s plants – potato tubers and daffodil bulbs are examples of this.
Artificial cloning in plants:
Cloning plants has many important commercial implications – it allows a successful variety of a plant to be produced cheaply in a short space of time and on a massive scale.
- Putting the cut end of a shoot into water or moist earth.
- Roots grow from the base of the stem into the soil while the shoot continues to grow and produce leaves.
- It uses tiny pieces from the parent plant, rather than cuttings.
- Sterile agar jelly with plant hormones and lots of nutrients are needed.
- Tissue culture is more expensive and more difficult than taking cuttings.
The advantages and disadvantages of asexual reproduction:
Sexual reproduction: is a process involving the fusion of two gametes (sex cells) to form a zygote and the production of offspring that are genetically different from each other.
Fertilisation: is the fusion of gamete nuclei.
Gametes are made in the reproductive organs.
The process of cell division that produces the gametes is called meiosis.
In sexual reproduction, the male and female gametes come together and fuse, that is, their cytoplasm and nuclei join together to form a single cell called a zygote.
- In flowering plants the male gametes are found in pollen grains and the female gametes, called egg cells are present in ovules.
- In animals, male gametes are sperm and female gametes are eggs.
- Male gamete is microscopic and mobile. The sperm swim to the ovum; the pollen cell moves down the pollen tube.
- The female gametes are always larger than the male and are not mobile.
- In normal body cells (somatic cells) the chromosomes are present in the nucleus in pairs.
- Humans have 46 chromosomes; 23 pairs. This is known as diploid
- When the gametes are formed, the number of chromosomes in the nucleus of each sex cell is halved. This is the haploid
- During fertilisation, when the nuclei of the sex cells fuse, a zygote is formed.
- It gains the chromosomes from both gametes, so it is a diploid cell.
The advantages and disadvantages of sexual reproduction:
- It is possible for biologists to use their knowledge of genetics to produce new varieties of plants and animals.
- A long-term disadvantage of selective breeding is the loss of variability, by eliminating all the offspring who do not bear the desired characteristics, many genes are lost from the population.
- At some future date, when new combinations of genes are sought, some of the potentially useful ones may no longer be available.
16.3) Sexual reproduction in plants
by A* Biology on December 7, 2016 in
16.3) Sexual reproduction in plants
- In the flower of most plants there are both stamen (male organs) and carpels (female organs), this is a condition known as bisexual or hermaphrodite.
- Some plants have unisexual flowers.
- Grasses have wind-pollinated flowers.
- They have small petals, and their stamens and stigmas hang outside the flower.
Pollination: is the transfer of pollen grains from the anther to the stigma.
Self-pollination: is the transfer of pollen grains from the anther of a flower to the stigma of the same flower, or a different flower on the same plant.
- No variation.
- Not be able to adapt to changing environmental conditions.
- No reliance on pollinators.
Cross-pollination: is the transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species.
- Guarantee variation.
- Better chance of adapting to changing conditions.
- Reliance on pollinators to carry the pollen to other plants.
- When a pollen grain lands on the stigma of a flower of the correct species, a pollen tube begins to grow.
- It grows down the style and into the ovary, where it enters a small hole, the micropyle, in an ovule.
- The nucleus of the pollen then passes along the pollen tube and fuses with the nucleus of the ovule.
- This process is called fertilisation.
Environmental conditions that affect germination of seeds:
Germination is a process, controlled by enzymes, in which the seed begins to develop into a new young plant. Three main factors are needed for successful germination.
16.4) Sexual reproduction in humans
by A* Biology on December 7, 2016 in
16.4) Sexual reproduction in humans
Male reproductive system:
- Testes: It is a male gland which produces sperms and the male sex hormone testosterone
- Scrotum: it is the sac which contains the testicles
- Sperm Ducts: They are two muscular tubes, each connected to a testis. They carry the sperms from the testis to the urethra
- Prostate Gland: It secretes a nutritive fluid to the sperms to form a mixture called semen
- Urethra: It is a tube inside the penis which is the pathway of semen and urine out of the body
- Penis: It is the male sex organ which ejaculates semen into the vagina during sexual intercourse
- Epididymis: coiled tubes in which sperms are stored
- Seminal vesicle: it is another gland like the prostate gland. It also secretes nutritive fluids for sperms to feed from and swim in forming semen
Female reproductive system:
- The Ovaries: They contain follicles where eggs are produced
- Oviducts (Fallopian Tube): They are two tubes, one on each side connected to an ovary. They are where fertilization occurs and they provide a pathway for the eggs to travel to the uterus by sweeping them by cilia on its walls
- Uterus (Womb) : Where the fetus develops,
- Cervix: A muscular tissue which separates the vagina from the uterus
- Vagina: it receives the male penis during sexual intercourse
Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete, producing a new cell called a zygote. This then matures into an embryo.
- Sperm are much smaller than eggs and are produced in much larger numbers.
- The tip of the cell carries an acrosome, which secretes enzymes capable of digesting a path into an egg cell, through the jelly coat, so the sperm nucleus can fuse with the egg nucleus.
- The cytoplasm of the midpiece of the sperm contains many mitochondria. They carry out respiration, providing energy to make the tail (flagellum) move and propel the sperm forward.
- The egg cell is much larger than sperm cell and only one egg is released each month while the woman is fertile.
- It is surrounded by a jelly coat, which protects the contents of the cell and prevents more than one sperm from entering and fertilising the egg.
- The egg contains a large amount of cytoplasm, which is rich in fats and proteins.
- The fats act as energy store. Proteins are available for growth if the egg is fertilised.
Pregnancy and development:
After fertilisation, the newly-formed zygote divides repeatedly to form a ball of cells called an embryo. This becomes implanted in the wall of the uterus.
After eight weeks of development, the embryo is called a fetus. The amniotic sac produces amniotic fluid, which surrounds and protects the developing embryo.
A placenta, connected by an umbilical cord, develops from the embryo.
The placenta anchors the embryo in the uterus. It also allows:
- nutrients and oxygen to move from the mother to the embryo
- waste materials and carbon dioxide to move from the embryo to the mother
There is no physical connection between the circulatory systems of the embryo and its mother, so their blood doesn’t mix. These materials pass from one to the other by diffusion.
- The placenta can prevent some harmful substances in the mother’s blood from reaching the embryo. It cannot prevent all of them, however: alcohol and nicotine can pass to the developing fetus.
- Some pathogens such as the rubella virus and HIV can pass across the placenta.
- The placenta produces hormones, including oestrogens and progesterone which are essential to keep the uterus in good condition and stimulate milk-producing tissues in the mother.
‘Antenatal” or “prenatal” refers to the period before birth. Antenatal care is the way a woman should look after herself during pregnancy, so that the birth will be safe and her baby healthy.
- Eat properly, take more iron and folic acid (a vitamin) to prevent anaemia.
- Drinking or smoking are more likely to cause babies with low birth weights. These babies are more likely to be ill than babies of normal birth weights.
Labour and birth:
- The period from fertilization to birth takes about 38 weeks in humans. This is called the gestation
- A few weeks before the birth, the fetus has come to lie downwards in the uterus, with its head just above the cervix.
- When birth starts, the uterus begins to contract rhythmically. This is the beginning of what is called ‘labour’.
- These regular rhythmic contractions become stronger and more frequent.
- The opening of the cervix gradually widens (dilates) enough to let the baby’s head pass through and the contractions of the muscles in the uterus wall are assisted by muscular contraction of the abdomen.
- The amniotic sac breaks at some stage in labour and the fluid escapes through the vagina.
- Finally, the muscular contractions of the uterus wall and abdomen push the baby head-first through the widened cervix and vagina.
- The umbilical cord, which still connects the child to the placenta, is tied and cut. Later, the placenta breaks away from the uterus and is pushed out separately as the ‘afterbirth’.
- The best food for a newborn is breast milk. This is because breast milk contains all essential nutrients for the baby like proteins, fats, sugars, vitamins and minerals in easily digestible form.
- The mother’s milk also contains antibodies which are needed by the baby since their immune system has not developed yet.
- Moreover, breastfeeding builds a close bond between the mother and her baby.
- There is no risk of an allergic reaction to breast milk.
- Breast milk is produced at the correct temperature.
- There are no additives or preservatives in breast milk.
- Breast milk does not require sterilisation since there are no bacteria present that could cause intestinal disease.
- There is no cost involved in using breast milk and does not need to be prepared.
- Breastfeeding triggers a reduction in the size of the mother’s uterus.
- If the mother cannot breastfeed for any reason, there is another alternative which is formula milk powder. Formula milk powder is mixed with boiled water and fed to the babies in bottles.
- Formula milk however contains nutrients in harder digestible form which is a disadvantage.
- Formula milk also lacks of antibodies which are needed by the baby which makes a bottle feeding baby in a greater risk of infection than a breastfeeding baby.
Sex hormones in humans
Puberty is the stage in life when a child’s body develops into an adult’s body. The changes take place gradually, usually between the ages of 10 and 16.
Changes occur at puberty because of hormones:
- testosterone – produced by the testes – controls the development of male secondary sexual characteristics
- oestrogen – produced by the ovaries – controls the development of female secondary sexual characteristics
The menstrual cycle:
- The ovaries release an ovum about every 4 weeks.
- In preparation for this the lining of the uterus wall thickens, so that an embryo can embed itself if the release ovum is fertilised.
- If no implantation occurs, the uterus lining breaks down. The cells, along with blood are passed out of the vagina. This is called a menstrual period.
- Several hormones control this cycle:
Methods of birth control in humans
Birth control is controlling the number of children and the time to have them. A couple may use birth control if they are not ready to have a baby yet. There are several types and methods of birth control. Types of birth control are natural, chemical, mechanical and surgical.
- Abstinence method is simple avoiding sexual inter course, this way there is no chance the woman will get pregnant.
- Rhythm method is based on the woman understanding her menstrual cycle (period). The woman must be able to sense and predict the time of ovulation when the egg is in the oviduct waiting to be fertilized, and not have sexual intercourse at that time. The woman can know when it is ovulation time of the cycle by seeing the type of mucus secreted by the cervix and lining of the vagina and a slight rise in body temperature
- Spermicides is a cream that contains a substance that will kill sperms. The cream has to be placed in the woman’s vagina before sexual intercourse so that it kills the sperms that will be ejaculated.
- Contraceptive pill is a pill which contains chemicals that prevent the ovaries releasing an egg to the oviduct (ovulation); there won’t be an egg ready for fertilization. In some cases, the pill has to be taken every single day, if it is forgotten once there is a chance of pregnancy. The pill is very effective, but it is not preferred by some women since it could bring other side effects such as mood swings, weight gain or circulatory diseases like strokes. The pill has to be prescribed by a doctor who performs a check up on the woman in advance.
- Intra-uterine device (IUD) is a small T-shaped plastic and copper device, inserted by a doctor into the wall of the uterus, where it probably prevents implantation of a fertilised ovum. There is a small risk of developing uterine infections.
- Intra-uterine system (IUS) is similar to IUD and also releases the hormone progesterone slowly over a long period of time ( up to 5 years). The hormone prevents ovulation.
- Contraceptive implant is a small plastic tube about 4 cm long, which is inserted under the skin of the upper arm of a woman by a doctor. Once in place it slowly releases the hormone progesterone, preventing pregnancy. It last for about 3 years.
- Contraceptive injection contains progesterone and stays effective for between 8 and 12 weeks. It works by thickening the mucus in the cervix. Stopping sperm reaching an egg. It also thins the lining of the uterus, making it unsuitable for implantation of an embryo.
- Condom is simply a layer of cover worn on the penis to keep semen from entering the woman’s body. It is made of stretchy, impermeable material that won’t allow any substance entering the woman’s body from the man or vise versa. The condom also has a great advantage; it also prevents diseases or bacteria and viruses passing from the man to the woman or vise versa.
- Femidom is a female condom that is worn by the woman instead of the man. It acts like a bag in the vagina in which the penis is inserted and the semen is ejaculated without entering the woman’s body.
- Diaphragm is a small, circular piece of rubber which is fit over the woman’s cervix to prevent sperms from passing through it. It is impermeable and it can be used along with spermicidal cream to ensure that sperms will not pass through the cervix.
- Male sterilisation – vasectomy: the man could have his sperm ducts cut and sealed, in this way the sperms won’t even leave his body.
- Female sterilisation – laparotomy: the woman could have an operation to cut and seal her oviducts to ensure that the egg can’t pass down to the uterus.
The use of hormones in fertility and contraception treatments:
Hormones to improve fertility:
- Failure to produce ova can be treated with fertility drugs.
- These drugs are similar to hormones and act by increasing the levels of FSH and LH.
- Administration of the drug is timed to promote ovulation to coincide with copulation.
Hormones for contraception:
- Oestrogen and progesterone control important events in the menstrual cycle.
- These hormones can be used, singly or in combination, in a range of contraceptive methods.
Artificial insemination (AI):
- If the male is infertile, not enough sperm or sperm not mobile enough, pregnancy may be achieved by AI.
- This involves injecting semen through a tube into the top of the uterus.
In vitro fertilisation:
- ‘’In vitro’ means literally ‘in glass’ (allowed to take place in laboratory glassware).
- Multiple ova caused by fertility drugs are collected by laparoscopy (they are sucked up in a fine tube inserted through the abdominal wall).
- The ova are then mixed with the husband’s seminal fluid and watched under the microscope to see if cell division takes place.
- One or more of the dividing zygotes are then introduced to the woman’s uterus by means of a tube inserted through the cervix.
Social implications of contraception and fertility treatments:
Some religions are against any artificial forms of contraception and actively discourage the use of contraceptives such as the sheath and femidom, However, these are important in the prevention of transmission of STDs in addition to their role as contraceptives.
Fertility treatments such as in vitro fertilization are controversial because of the ‘spare’ embryos that are created and not returned to the uterus. Some people believe that since these embryos are potential human beings, they should not be destroyed or used for research. In some cases the ‘spare’ embryos have been frozen and used later of the first transplant did not work.
Sexually transmitted infections (STIs)
Sexually transmitted infection is an infection that is transmitted via body fluids through sexual contact.
AIDS and HIV:
- Acquired immune deficiency syndrome. ( A ‘syndrome’ is a pattern of symptoms associated with a particular disease.)
- The virus that causes AIDS is the human immunodeficiency virus.
- HIV is transmitted by direct infection of the blood.
- Drug users who share needles contaminated with infected blood run a high risk of the disease.
- It can also be transmitted sexually.
- Babies born to HIV carriers may become infected with HIV.
- If HIV antibodies are present in the blood, the person is said the be HIV positive.
Control of the spread of STIs:
- The best way to avoid STI is to avoid having sexual intercourse with an infected person.
- The risk of catching a STD can be reduced by using condoms or femidoms.
- STIs that are caused by a bacterium, such as syphilis and gonorrhoea, can be treated with antibiotics if the symptoms are recognised early enough. However, HIV is viral so antibiotics are not effective.
The effects of HIV on the immune system:
- HIV attacks certain kinds of lymphocyte, so the number of these cells in the body decreases.
- Lymphocytes produce antibodies against infections. If the body cannot respond to infections through the immune system, it becomes vulnerable to pathogens that might not otherwise by life-threatening.
- As a result, the patient has little or no resistance to a wide range of diseases such a influenza, pneumonia, blood disorders, skin cancer or damage to the nervous system, which the body cannot resist.